The present invention relates to a permanent magnet assembly for stably fixing a denture by utilizing a magnetic attraction force of a permanent magnet.
Various attempts have already been made to fix a denture by utilizing an attraction force of a permanent magnet to a soft-magnetic alloy. One example of such attempts is described in "Application of Rare Earth Magnets to Osseo-Integrated Implant," T. R. Jackson, Oral Maxillofacial Implant, Vol. 1, No. 2 (1987), pp. 77-89.
To practically use such a denture, it is necessary that a permanent magnet is completely sealed in a member made of a material whose harmlessness to human body is fully verified, and that there is little magnetic flux leaking outside from the permanent magnet member. For instance, a permanent magnet assembly having a constitution as shown in FIG. 9 has been used. In FIG. 9, (a) is a schematic perspective view showing the permanent magnet assembly, and (b) is a schematic view showing the cross section of the permanent magnet assembly. A cylindrical rare earth-cobalt magnet 1 axially magnetized as shown by the arrow is received in a cylindrical casing 2 made of a magnetic stainless steel alloy having excellent corrosion resistance, and covered by a thin disc plate 3 made of a non-magnetic stainless steel alloy having excellent corrosion resistance, these members being bonded to each other by an adhesive 4 to seal the inside of the casing 2.
Such a permanent magnet assembly constituted by the permanent magnet 1, the casing 2 and the disc plate 3 is buried in a denture plate 8 for fixing the denture as shown in FIG. 10. Buried in an alveolus 9 is a root member 7 made of a soft-magnetic alloy as shown in FIG. 10. When the magnet assembly is placed on the root member 7 with the non-magnetic disc plate 3 of the magnet assembly facing the root member 7, a magnetic attraction force is generated between the magnet assembly and the root member 7. By this attraction force, the denture plate 8 is fixed to the alveolus 9. At this time, the magnetic flux generated from the magnet 1 flows through a circular course from the magnet 1 to the root member 7, the casing 2 and then to the magnet 1. Accordingly, there is extremely little magnetic flux leaking outside from the permanent magnet assembly. Also, since the casing 2 and the disc plate 3 are made of stainless steel materials, they are corrosion-resistant and show practically sufficient wear resistance when they are used for fixing the denture and subjected to dental articulation. With respect to the adhesive 4, it has been considered to be sufficient in sealability and chemical stability when used in the mouth.
However, the inventors have found that the permanent magnet assembly having the above structure is disadvantageous in that since the adhesive 4 is made of an organic material it is still insufficient in adhesion strength and chemical stability for a long period of use despite its practically sufficient properties. Also, despite the fact that the conventional permanent magnet assembly of FIG. 9 is excellent in preventing the leakage of magnetic flux, it fails to provide a sufficient attraction force. Accordingly, a more reliable permanent magnet assembly of a seal type capable of fixing the denture stably with a larger attraction force is desired.